Line echoes (i.e., electrical echoes) occur in telecommunications networks due to impedance mismatches at hybrid transformers that couple two-wire local customer loops to four-wire long-distance trunks. Ideally, the hybrid passes the far-end signal at the four-wire receive port through to the two-wire transmit port without allowing leakage into the four-wire transmit port. However, this would require exact knowledge of the impedance seen at the two-wire ports, which in practice varies widely from individual circuit to individual circuit and can only be estimated. Consequently, the leaking signal returns to the far-end transmitter as an echo. The situation can be further complicated by the presence of two-wire toll switches, allowing intermediate four-two-four wire conversions internal to the network. In telephone connections using satellite links, with round-trip delays on the order of 600 ms, line echoes can become particularly disruptive.
Echo suppressors have been developed to control line echoes in telecommunications networks. Echo suppressors de-couple the four-wire transmit port when signal detectors determine that there is a far-end signal at the four-wire receive port without any near-end signal at the two-wire receive port. Echo suppressors, however, are generally ineffective during double-talk when speakers or modems at both ends of the system are talking or transmitting simultaneously. During double-talk, the four-wire transmit port carries both the near-end signal and the far-end echo signal. Furthermore, echo suppressors tend to produce speech clipping, especially during long delays caused by satellite links.
Echo cancellers have been developed to overcome the shortcomings of echo suppressors. Echo cancellers were first deployed in the U.S. telephone network in 1979, currently are virtually ubiquitous in long-distance telephone circuits. See generally, Messerschmitt, “Echo Cancellation in Speech and Data Transmission”, IEEE Journal on Selected Areas in Communications, Vol. SAC-2, No. 2, March 1984, pp. 283–298; and Tao et al., “A Cascadable VLSI Echo Canceller”, IEEE Journal on Selected Areas in Communications, Vol. SAC-2, No. 2, March 1984, pp. 298–303.
Echo cancellers generally include an adaptive filter and a subtracter. The adaptive filter attempts to model the echo path. The incoming signal is applied to the adaptive filter, which generates a replica signal. The replica signal and the echo signal are then applied to the subtracter. The subtracter removes the replica signal from the echo signal to produce an error signal. The error signal is fed back to the adaptive filter, which adjusts its filter coefficients (or taps) in order to minimize the error signal. In this manner, the filter coefficients converge toward values that optimize the replica signal in order to cancel (i.e., at least partially offset) the echo signal. Echo cancellers offer the advantage of not disrupting the signal path. Economic considerations place limits on the fineness of sampling times and quantization levels in digital adaptive filters, but technological improvements are relaxing these limits.
In the field of data communications a transceiver, or modem, is used to convey information from one location to another. Digital subscriber line (DSL) technology now enables DSL transceivers to more rapidly communicate data than previously possible with purely analog modems. DSL transceivers communicate by modulating a baseband signal carrying encoded digital data, converting the modulated digital data signal to an analog signal, and transmitting the analog signal over a conventional copper wire pair using techniques that are known in the art. These known techniques include mapping the information to be transmitted into a multi-dimensional multi-level signal space constellation and slicing the received constellation to recover the transmitted information. The constellation can include both analog and digital information or only digital information.
Since DSL transceivers use the public switched telephone network (PSTN) and other similar networks to communication over twisted-wire pairs, DSL systems are subject to the same echo cancellation problems described above with respect to voice band users of the PSTN. Echo is very harmful to successful DSL signal delivery as it significantly degrades signal quality. Therefore, an echo cancellation technique should be employed to separate the upstream and downstream signals in DSL transceivers.
Various signal-processing techniques may be used to cancel the echo. Often, the echo signal has a large low-frequency content that results in an echo response in the time-domain with a very long tail. Under these conditions, an echo canceller with many taps is required to effectively cancel the echo. An echo canceller with many taps requires a large amount of signal processing resources and results in a significant computation cost. One technique that reduces low frequency content of the echo signal is to insert a high pass filter in the receive path. This technique, however, removes useful receive signal energy (i.e., data) resulting in reduced receiver performance.
As previously described, echo occurs primarily because of mismatched impedances at the hybrid connectors. Because the impedances of the transmission lines are time variant as well as line-dependent, the echo canceller must be adaptive. Stated another way, the echo canceller must learn the echo characteristics and track time-varying changes in the underlying communication system.
The echo canceller can be made adaptive as follows: After estimating the echo parameters, emulating the echo, and subtracting it from the received signal, the remaining signal (which contains some residual echo) may be fed back and used to update the estimated set of echo parameters. This feedback loop allows the echo canceller to converge to a close approximation of the echo parameters.
Echo cancellation may be accomplished either in the time-domain or in the frequency-domain. In time-domain echo cancellation, echo parameters are derived through use of, for example, a gradient search algorithm, which adjusts the echo parameters in order to minimize some performance criteria (for example, mean square error). In frequency-domain echo cancellation, the echo parameters are obtained by sampling the spectrum of the echo channel rather than sampling the echo channel response. Echo emulation and adaptive updates can then take place in the frequency-domain, using the estimate of the spectrum of the echo channel.
Most existing echo cancellers use the tapped-delay line structure (also known as a finite impulse response (FIR) filter) to model and replicate the echo. However, at high data rates, FIR filters can be several hundred taps long, and the computational complexity can become extremely high. As a result, digital filters generally perform a large number of computations (e.g., multiplication) and require a large number of storage elements (e.g., registers) for temporarily storing computed variables (e.g., state variables) in carrying out the computations. The number of storage elements and computations carried out by a digital filter directly effects the device's size, speed, and power consumption. As the complexity of digital filter increases, the number of computations and the number of registers required tend to increase. Particularly, in systems utilizing baseband modulation schemes, including Pulse Amplitude Modulation (PAM), where there is a large low frequency signal content and, consequently, a long echo tail. For these reasons, it is desirable to improve the computational efficiency and speed (e.g., reduce the number of computations) and reduce the number of storage elements required in FIR filters. Accordingly, an alternative solution that overcomes the shortcomings of the prior art is desired.